1. Field of the Invention
This invention generally relates to dental porcelain restorations and methods of manufacture thereof. In particular, this invention relates to a color dental porcelain system and a method of use of the system for accurately matching the color of a porcelain restoration to a patient's natural tooth color.
2. Brief Description of the Related Art
In the restoration of a tooth or a set of teeth, the tooth color must be correctly selected. A tooth restoration should not only fit harmoniously into the arc of the teeth but it should also be adapted to the individual conditions of the adjacent teeth and the entire set of teeth in its course of colors.
Tooth color is affected by a number of factors. The inherent color of the restoration is important, as well as the spectral composition of the particular light source with which the restoration is illuminated. The natural tooth is not opaque, but more or less translucent on its surface; for this reason, the transparencies of the enamel and of the dentin need to be taken into consideration. Light reflections result in bright spots and bright lines, so even the surface nature of the replacement tooth must agree with the surface of the adjacent teeth. It has long been known in the dental profession that the good aesthetics of dental prostheses are to a significant degree determined by the hue, the chroma, and the value of the visible exposed portions of the dental restoration. Translucency is also an important color parameter.
As used herein hue is a color or the name of a color, for example, blue, red, orange, violet, green, and so forth. A hue may be a primary color or the result of a combination of colors, for example, greenish-blue or orangish-yellow. The hues of primary interest in dental restorations are red and yellow. Chroma as used herein refers to the strength, intensity, or amount of saturation of the hue. Value as used herein refers to the brightness of a color in the range of white to black. Any point in between white and black will comprise a certain intensity of gray. For example, a darker gray color will have a low value, while a lighter gray will have a higher value. Value is one of the more important factors in proper tooth matching, as a tooth of high value is generally bright and vital, whereas a tooth with a low value has a non-vital, gray appearance. Translucency as used herein is the quality of transmitting and diffusing light, which gives a perception of depth, and is expressed in the range from transparent to opaque.
The parameters of hue, value, and chroma are often presented in the form of a three-dimensional array having a uniform color scale. One commonly-used system employs the CIE (Commission Internationale de l'Eclairage [International Commission of Illumination]) L*a*b conventions, which represents colors in a three-dimensional Cartesian co-ordinate system. The vertical axis represents luminance or lightness "L", which is equivalent to value. The horizontal co-ordinates represent chromaticity, and are designated "a" (positive representing red and negative representing green) and "b" (positive representing yellow and negative representing blue). Thus, the positive and negative portions of the horizontal axes represent opposed colors of increasing chroma moving way from the axes. Hue is calculated mathematically as tan.sup.-1 (b*/a*) Chroma is calculated as ([a*.sup.2 +b*.sup.2 ]).sup.1/2. A variant of this co-ordinate system expresses the rectangular co-ordinates "a" and "b" as polar coordinates "C" and "h", where "C" is the magnitude component representing chroma and "h" is the angular component representing hue. Value or lightness if plotted on the z-axis through the sphere.
Hue, chroma, value, and translucency define the color components found individually in varying degrees and cooperatively in varying ratios in every tooth color. However, a proper interpretation and accurate communication of the varying degrees and ratios of these parameters is difficult. A common practice in the dental arts in the selection of colors is to employ a dental shade guide to assist in the matching of a patient's natural tooth color. A typical prior art shade guide includes an elongated, linear holder containing a plurality of specimens. Each of the specimens is comprised of a sample tooth and a support member for attaching the sample tooth to the holder. Each sample tooth is provided with a predetermined shade value, chroma, and hue for matching with the patient's natural teeth.
The dentist places the shade guide teeth in front of the patient's natural teeth to select a shade most closely approximating the coloring of the patient's natural teeth. Then the dentist, often together with the patient, typically selects a hue and shade progression according to the age of the patient. After the best color match has been selected, the dentist communicates this coloring to the dental ceramist, who then constructs the dental restoration by selecting appropriate porcelain powders. The selection of the porcelain powders is usually dictated by the letter and number code that is communicated to the dental ceramist.
One drawback with existing systems is the trial-and-error nature of trying to match the dental restoration color to the patient's teeth. While the colors of teeth appear to range from a light almost white-tan to a light brown, the colors involved may actually contain a small amount of nearly any color of the visual spectrum. Even a slight variation in color will become apparent when the dental restoration is positioned closely adjacent to natural teeth. Teeth appear to be different colors under different illumination systems (e.g., natural vs. fluorescent light) and light levels, which further complicates shade selection. In addition, the color of an individual tooth is not evenly and uniformly distributed. The color, brightness, and transparency decrease from the cutting edge or the masticatory surface of a tooth to the dental neck. The enamel exhibits a whitish-bluish color and the dentin exhibits a yellowish to brownish color tones. The dental neck and the root element are likewise of a yellowish-brownish color but usually are darker than the corresponding dentin. The canines are usually darker than the other teeth.
Existing color shade guides and the porcelain systems used with them typically do not allow a dentist or technician to independently increase or decrease each one of the color components (e.g., translucency) without affecting the other three color components (e.g. hue, chroma, or value) for each of the color parameters. A shade guide system presently in widespread use is the guide manufactured by Vita Zahnfabrik under the trade name Vita Lumen.RTM. Vacuum Shade Guide. As shown in FIG. 1, the shades are somewhat arbitrarily distributed within color space.
Attempts have been made to allow greater flexibility in shade matching, but none is at present entirely satisfactory. For example, U.S. Pat. No. 4,207,678 to Jeannette discloses a dental shade guide system having a plurality of primary shade guides having a specific chroma percentage, and each primary shade guide having a corresponding plurality of secondary shade guides such that each of the secondary shade guides has a decreased chroma percentage. The decreased chroma percentage in each secondary shade guide is accomplished by mixing the primary shade guide formula in varying amounts with a gray or white modifier. A primary shade guide is first selected and then a color selection is made from the corresponding secondary shade guide.
U.S. Pat. No. 5,240,414 to Thompson discloses a method of shade selection which uses a plurality of shade tabs to simulate color reproduction and/or tooth shade. An opaque porcelain tab having preselected characteristics is first selected; then, a dentin porcelain tab of preselected characteristic is selected and interfitted upon the opaque tab; next, an enamel tab of preselected characteristics is selected and interfitted upon the dentin tab. U.S. Pat. No. 3,964,167 to Yerkes discloses a disposable tooth shade guide having an artificial tooth of a predetermined shade mounted on a holder with a cap or covering detachably mounted over the artificial tooth. The cap/cover is colored a predetermined shade whereby when it is mounted on the artificial tooth, it alters the basic color of the artificial tooth to produce a variation in shade. Each of these above-described methods is limited in that only one of the four color parameters is addressed in determining proper tooth color.
In contrast, U.S. Pat. No. 5,529,492 to Yarovesky et al. addresses the problem that conventional shade guides do not permit the dentist to inform the laboratory regarding the characteristics to be incorporated into the prosthesis, for example differences between the neck portion and the incisal portion of the tooth, or cracks or other localized discolorations. This patent discloses an anterior tooth characterization guide which includes several sets of anterior tooth samples which have differing, independently selectable color characteristics including the dentin color, the incisal color, the body:incisal relation, the dentin structure and translucent effect, characterizations, and amount of white stain. The guide is utilized to first select a dentin color (keeping incisal color constant), the incisal color (keeping dentin color and blend constant), and the body to incisal relation (keeping dentin color constant). The dentin structure and translucent effect are then determined holding the dentin color constant, additional characterization features including cracking, spotting, and hallow are then selected holding dentin color constant, and finally, the amount of white stain is determined holding the dentin, enamel and blend constant. While this method holds various aspects of tooth coloration constant while varying others to enable a choice of a particular prosthesis color and characteristic, it is a multi-step, time-consuming process. It furthermore fails to address the formation of an appropriate prosthesis and does not allow independent variation among hue, value, and chroma for each of the tooth parts.
This deficiency is at least partially remedied by the systems disclosed U.S. Pat. Nos. 5,498,157 and 4,657,399 to Hall, which are incorporated herein by reference. Hall discloses dental color systems comprising arrayed color samples which coincide with an evenly-spaced, corresponding location on a CIE L*a*b color co-ordinate system. As disclosed by Hall, a set of dental shade guides is constructed based first on selection of two or more evenly spaced value or lightness levels. For each value or lightness level, a set of seven shades is provided having various a, b (hue and chroma) values. The a, b values within the set correspond to the intersection points of a grid as laid out on the a, b axes. The size, placement, and number of intersections of grid is preferably selected so as to correspond to the majority (&gt;95%) of the range of hue and chroma values of human teeth as determined by Hall. The distance between any two adjacent colors along the b-axis is therefore about 5 units, while the difference between adjacent colors along the a-axis is about 2.5 units. Each grid preferably has a central color, which is offset from the central color in the adjacent level by 2-4 degrees of hue towards the red for each 4-6 unit decrease in luminance.
Although Hall represents an advance over prior art systems for judging and communicating tooth color, it still has a number of drawbacks and disadvantages. One of these is the necessity for the arbitrary selection of a set lightness, hue, and value parameters for the manufacture of the shade guide and of the porcelain. While Hall has systematized selection of these parameters to some extent, the particular shades provided to the practitioner still remains arbitrary, and will not match all tooth colors. The practitioner is forced to estimate the approximate distance between two adjacent colors in these instances.
Another drawback to the Hall system is that there is no provision for translucency. Translucency has been particularly difficult to reproduce accurately. Natural teeth, as well as properly made dental restorations, are to some extent translucent so that the color of the back of a tooth, particularly the back of the lower part of the incisal portion of a tooth may be as important as the color of the front or normally exposed portion of a tooth. Within a tooth, there may be regions of varying translucency. Furthermore, the translucency of teeth increases as people age. U.S. Pat. No. 5,114,340 to Hahn discloses a platelet color specimen having at least one lateral edge concavely curved towards the center region, wherein the specimen is on the order of a width of a tooth. The platelet color specimen exhibits only a single hue having varying transparency corresponding to enamel, dentin, or dental neck. As with other color guides, however, the range of colors and other variables is limited.
In addition to the above described drawbacks, the color systems of the prior art do not provide accurate color matching because of human error, i.e., the doctor or fabricating technician may misread the color number on the shade guide or the shade guide manufacturer may erroneously identify the shade of one or more specimens in the shade guide. There may be inconsistency between the laboratory and dentist in the type of shade guide. The tooth manufacturer may slightly change the shades of teeth or tooth powder in its production batches because of changes, which frequently depend upon availability, of one or more ingredients.
Another source of error arises because shade guides age, which changes the apparent colors of the guide. For example, the color intensity is reduced and colors tend to lighten or fade with time. This is particularly true where the shade guide is overexposed to sunlight, frequently disinfected, or sterilized, resulting in a change in the original tones. Finally, it is noteworthy that the tooth portions of shade guides are often manufactured from acrylic or similar materials, rather than porcelain. The differences between the shades achievable in plastics and those of porcelain provide further subtle inaccuracies. The occurrence of any of these events will result in an inaccurate matching of artificial teeth shade with the shade of natural teeth because the technician normally does not have access to the patient and was not present when the dentist made the initial shade selection.
Accordingly, there remains a need in the dentistry field for a color system for porcelain dental restorations in which the color of the porcelain restoration is accurately measured and communicated to the laboratory, and porcelain powders can be mixed in accordance with the reported values to achieve a restoration matched to the color of the patient's natural teeth. In particular, there remains a need for a system which the color components (hue, chroma, value, and translucency) of a patient's tooth are independently taken into account when selecting the proper color of a porcelain dental restoration, and are not limited by an arbitrary selection of colors within the relevant color space.